Potassium channels are involved in a number of physiological processes, including regulation of heartbeat, dilation of arteries, release of insulin, excitability of nerve cells, and regulation of renal electrolyte transport. Potassium channels are thus found in a wide variety of animal cells such as nervous, muscular, glandular, immune, reproductive, and epithelial tissue. These channels allow the flow of potassium in and/or out of the cell under certain conditions. For example, the outward flow of potassium ions upon opening of these channels makes the interior of the cell more negative, counteracting depolarizing voltages applied to the cell. These channels are regulated, e.g., by calcium sensitivity, voltage-gating, second messengers, extracellular ligands, and ATP-sensitivity.
Potassium channels are made by alpha subunits that fall into 8 families, based on predicted structural and functional similarities (Wei et al., Neuropharmacology 35(7):805–829 (1997)). Three of these families (Kv, eag-related, and KQT) share a common motif of six transmembrane domains and are primarily gated by voltage. Two other families, CNG and SK/IK, also contain this motif but are gated by cyclic nucleotides and calcium, respectively. The three other families of potassium channel alpha subunits have distinct patterns of transmembrane domains. Slo family potassium channels, or BK channels have seven transmembrane domains (Meera et al., Proc. Natl. Acad. Sci. U.S.A. 94(25):14066–71 (1997)) and are gated by both voltage and calcium or pH (Schreiber et al., J. Biol. Chem. 273:3509–16 (1998)). Another family, the inward rectifier potassium channels (Kir), belong to a structural family containing 2 transmembrane domains, and an eighth functionally diverse family (TP, or “two-pore”) contains 2 tandem repeats of this inward rectifier motif.
Potassium channels are typically formed by four alpha subunits, and can be homomeric (made of identical alpha subunits) or heteromeric (made of two or more distinct types of alpha subunits). In addition, potassium channels made from Kv, KQT and Slo or BK subunits have often been found to contain additional, structurally distinct auxiliary, or beta, subunits. These subunits do not form potassium channels themselves, but instead they act as auxiliary subunits to modify the functional properties of channels formed by alpha subunits. For example, the Kv beta subunits are cytoplasmic and are known to increase the surface expression of Kv channels and/or modify inactivation kinetics of the channel (Heinemann et al., J. Physiol. 493:625–633 (1996); Shi et al., Neuron 16(4):843–852 (1996)). In another example, the KQT family beta subunit, minK, primarily changes activation kinetics (Sanguinetti et al., Nature 384:80–83 (1996)).
Slo or BK potassium channels are large conductance potassium channels found in a wide variety of tissues, both in the central nervous system and periphery. They play a key role in the regulation of processes such as neuronal integration, muscular contraction and hormone secretion. They may also be involved in processes such as lymphocyte differentiation and cell proliferation, spermatocyte differentiation and sperm motility. Three alpha subunits of the Slo family have been cloned, i.e., Slo1, Slo2, and Slo3 (Butler et al., Science 261:221–224 (1993); Schreiber et al., J. Biol. Chem., 273:3509–16 (1998); and Joiner et al., Nature Neurosci. 1:462–469 (1998)). These Slo family members have been shown to be voltage and/or calcium gated, and/or regulated by intracellular pH.
A BK (or Slo) beta subunit that associates with Slo/BK potassium channels has been cloned and called BK beta 1 (Dworetzky et al., J. Neurosci. 16:4543–50 (1996); U.S. Pat. No. 5,776,734; see also Xia et al., J. Neurosci. 19:5255–5264 (1999), Wallner et al., Proc. Nat'l Acad. Sci. USA 96:4137–4142 (1999), Ali Riazi et al., Genomics 62:90–94 (1999), and EP 0 936 271 A1). BK beta 1 has short cytoplasmic N and C termini and has two membrane-spanning regions with a large extracellular loop. Functionally, BK beta 1 modulates the activation kinetics and calcium sensitivity of Slo1 channels (McManus et al., Neuron 14:645–50 (1995)). BK beta 1 also increases calcium sensitivity, sensitivity to extracellular toxins, and decreases the activation rate for Slo1 channels.
Additional beta subunits for Slo family potassium channels remain to be identified. The discovery and characterization of those Slo or BK beta subunits will provide important insights into how Slo potassium channels function in different environments, and how they respond to various activation mechanisms. Such information would also allow the identification of modulators of Slo potassium channels and the use of such modulators as therapeutic agents.